Exhaust Gas Recirculation (EGR): Exhaust Gas Recirculation System. Became common by 1973.
On an EGR engine, there are extra exhaust ports inside the cylinder heads that exhaust gases flow back into the air/fuel intake stream. The flow of these gases is controlled with a vacuum actuated piece called an EGR valve. It completes a connection between the intake manifold’s EGR passages and air/fuel passeges.
The EGR valve itself sits on the top the intake manifold, and is connected to a manifold vacuum port. Controlling this connection is a coolant temperature sensor that will only let vacuum through when the engine is properly warmed up.
The advantage of this system is a lesser amount of ‘oxides of nitrogen’ (NOx) get released from the tailpipe. Some engines actually will actually run cooler with an EGR valve too.